The present invention relates to a method and apparatus for injection molding multi-component mixtures, in particular liquid silicone rubber.
Silicone rubber is a generic term for a particular elastomer based on siloxane and has a chemical structure which is characterized by the siloxane chain (—Si—O—Si—O—), whereby organic methyl groups, phenyl groups, hydroxyl groups or the like are bounded as ligands. A distinction is made between solid silicone rubbers and liquid silicone rubbers (LSR) whereby the difference is characterized by their chain length. While liquid silicones are composed of a union of up to 1000 molecules, solid silicones contain a union of up to 6000–10000 molecules. As far as silicone cross-linking is concerned, two groups are distinguished: the RTV type (cross-linked at room temperature), and the HTV type (cross-linked at high temperature), i.e. cross-linked under hot-vulcanization condition.
The present invention relates primarily to the area of liquid silicones of the HTV type which become increasingly more relevant in the field of injection molding. This type of liquid silicones are two-component mixtures made of low viscous vinyl—or hydrogen-functional polysiloxanes with fillers and small quantities of a platinum catalyst which react according to the principle of the addition cross-linkage. The term “addition cross-linkage” denotes hereby a reaction of educts with one another under the influence of a catalyst to produce the end product, without formation of a cleavage product. These mixtures contain further an inhibitor, also called pot life regulator, to influence the period until commencement of cross-linkage. The inhibitor is added to one component whereas the catalytic cross-linkage agent is added to the other component of the two-component mixtures. This means in the context of liquid silicones that hydrogen-functional polysiloxane bonds with vinyl-functional polysiloxane under the influence of the catalyst which impacts only the reaction speed and is still in original state after the reaction.
Another method for making a two-component mixture involves peroxide cross-linkage where organic peroxides dissociate into free radicals as a consequence of the instability of the O—O bond under the influence of heat. The radicals react with the vinyl groups or methyl groups of the polysiloxane structure while forming hydroxyl compounds and enable in this way cross-linkage that leads to the formation of an atomic union between the individual monomers.
Turning now to FIG. 1, there is shown a conventional injection molding machine for injection molding formed parts made of liquid silicone rubber. Reference is also made in this context to German publication no. DE 34 003 09 A1. The injection molding machine includes a metering assembly 1 which has a storage tank 2 for containing component A, a storage tank 3 for containing component B as well as a storage tank 4 for containing an additive or a coloring agent F. A motor 5 drives barrel pumps 6, 7, 8 to transport the starting components A, B and the additive (or coloring agent) F through tubular conduits 9, 10, 11 via respective check valves 12, 13, 14 and a mixing head 15 to a static mixer 16. The actual mixing of the components is implemented in the static mixer 16 by means of flow dividing elements by which the material stream is layered many times. Subsequently, the mixture is fed via a shut-off valve 17 and a fitting 18 into the feed zone 19 of a plastification screw 20 of a plastification and injection unit 21. The rotary motion of the plastification screw 20 moves the material to an antechamber in front of the screw 20 in order to improve a mixing of the components through dynamic mixture. The screw cylinder is equipped with a water-based temperature-moderating structure to prevent excessive heating of the mixture. During subsequent injection operation, the screw 20 operates as plunger which forces material via a, not shown, cold runner system into a molding tool 22. Backflow of the very low-viscous mixture from the antechamber into the leading screw space is prevented by a particular backflow valve (LSR design) 23. Addition cross-linkage is realized in cavity 24 as soon as the required activation energy is exceeded as a result of a heating of the mixture. Vulcanization is effected at a temperature of about 170–200° C. and at cross-linking speeds of about 5 s/mm wall thickness of the formed part.
A main field of application for liquid silicone rubbers are small items such as, e.g., baby bottle nipples, mouth pieces of anesthetic equipments, and seals for various purposes, e.g., seal for a plunger of a syringe in the medical field or O rings in the automobile industry and engineering.
Conventional metering assemblies convey both components A and B at a particular, adjustable pressure from the storage tanks 2, 3, through the static mixer 16 directly into the plastification and injection unit 21 of the injection molding machine. As the distance traveled by the material during transport between the storage tanks 2, 3 and the plastification and injection unit 21 is oftentimes very long, the pressure may fluctuate in the area of the feed zone of the plastification and injection unit so that the material intake is uneven, resulting in metering variations or variations of the shot weight. In particular, when smallest amounts of liquid silicone are processed, problems can be encountered that adversely affect the overall quality of the molding result because injection of material may be insufficient or may even be excessive. Another problem involves the very small throughput of the components through the static mixer as a consequence of the small shot weights because the components are not thoroughly mixed enough.
It would therefore be desirable and advantageous to provide an improved method and apparatus for injection molding liquid silicone rubber or other mixtures comprised of several components, to obviate prior art shortcomings and to allow a thorough mixture of the components while avoiding pressure fluctuations in the area of the feed zone of the plastification and injection unit.